1. Field of the Invention
The present invention relates to railbound vehicles with wheels, and specifically to wheels used in such vehicles which include a wear layer containing a friction-increasing material intended to achieve more constant friction ratios between the wheel and the rail which it contacts. The invention is directed both to solid wheels, i.e., wheels made in one piece, and to built-up wheels which include a wheel centre and a tire with a tread a flange positioned around the wheel centre.
2. l The Prior Art
The amount of friction which occurs between the wheel of a railbound vehicle and the rail upon which it moves is known to vary considerably depending on the weather and the surface coating on the rail, such as oil or water and solid substances, which forms a lubricating layer on the rail.
The friction may vary with time and space because of rapid changes of factors influencing the friction. The coefficient of friction may vary considerably and may be between 0.07 and 0.7. In case of a high coefficient of friction, the tractive and braking forces are good but the track forces are great, the flange wear great and the running properties less good. In case of a low coefficient of friction, the risks of slipping increase and the wheels may be easily locked upon braking so that the wheels will become non-round. Since a high friction is desirable between the tread of the wheel and the rail, and a low friction is desirable between the wheel flange and the rail, attempts have been made to influence the friction so that the desired friction ratios are obtained. The classical method of increasing the coefficient of friction between the wheel and the rail is by sanding the rail. The sand grains break through the friction-reducing layers and are pressed into the materials of the wheel and the rail at the point of contact, thus improving the adhesion. It is also known to improve the adhesion by coating the tread with a thin layer of a friction-increasing material, but the hitherto known coatings have been easily worn out. It has therefore been necessary to renew the layers at short intervals. It is also known to heat-treat the tread of the wheel and the wheel flange differently, so that greater friction is obtained between the wheel tread and the rail than between the flange and the rail.